A MIMO scheme relates to multiple antenna based communications designed to improve the speed and/or quality of transmitted signals by using multiple antennas in communications. Downlink communications with multiple users in accordance with the MIMO scheme is called a “downlink multiple user MIMO” scheme or a “SDMA (Spatial Division Multiplex Access)” scheme. In the downlink multiple user MIMO scheme, a precoding scheme is utilized. The precoding scheme is a technique for transmitting signals to communication counterparts with directivity controlled beams by copying a transmission signal stream, synthesizing each of the copied streams with an appropriate weight and transmitting the resulting streams. The weights for use in the precoding scheme may be referred to as transmission weights, precoding vectors or precoding matrices.
FIG. 1 schematically illustrates a typical precoding operation. Each of two transmitted streams 1, 2 is duplicated at a copying unit, and precoding vectors are multiplied with the duplicated streams. After the resulting streams are synthesized with each other, the synthesized streams are transmitted. Stream S1 is transmitted to a first user apparatus UE1, and stream S2 is transmitted to a second user apparatus UE2. The precoding vectors are controlled adaptively to be more appropriate values based on feedbacks from the receiver side (user apparatuses). The precoding technique is described in detail in a document 3GPP R1-070236, “Precoding for E-UTRA downlink MIMO”, LG Electronics, Samsung and NTT-DoCoMo, for example.
Meanwhile, in the next generation mobile communication systems such as LTE (Long Term Evolution) systems being successors of the third generation mobile communication systems, one or more resource blocks or resource units are assigned to user apparatuses in both uplinks and downlinks for communications. The term “user apparatus” used herein includes not only a mobile station but also a fixed station. The resource blocks are shared among a large number of user apparatuses in a system. Channels shared in downlinks are referred to as PDSCHs (Physical Downlink Shared Channels). On the other hand, channels shared in uplinks are referred to as PUSCHs (Physical Uplink Shared Channels). A base station apparatus (eNB) determines which resource block is assigned to which of multiple user apparatuses in what transmission format for each subframe (or transmission time interval (TTI)) corresponding to 1 ms, for example. This process is called scheduling. The transmission format is determined through specifications for a data modulation scheme and a data size (or a channel coding scheme), and a scheme for appropriately modifying the transmission formats is called an AMC (Adaptive Modulation and Coding) scheme. In the downlink, the base station apparatus uses one or more resource blocks to transmit a PDSCH to a user apparatus selected through the scheduling. In the uplink, a user apparatus selected through the scheduling uses one or more resource units to transmit a PUSCH to the base station. Information regarding the uplink scheduling and the downlink scheduling is transmitted to the user apparatuses in each subframe (signaling). Control channels used for this signaling are referred to as PDCCH (Physical Downlink Control Channels) or DL-L1/L2 control channels.
The determined transmission formats and the radio resource assignment are modified depending on radio transmission states (channel states). The downlink channel state is measured at user apparatuses and periodically transmitted to the base station, for example, as a CQI (Channel Quality Indicator). The uplink channel state is measured at the base station based on reception quality of an uplink sounding reference signal (UL Sounding RS). The determined transmission formats and the radio resource assignment may directly affect system throughput. Thus, it is desirable that the base station can comprehend the channel states as accurately as possible.
Meanwhile, in the CQI measurement in the user apparatuses, the reception quality of downlink reference symbols (DL-RSs) may be represented as one of several tens of levels of reception SINR (Signal to Interference Noise Ratio), for example, and the CQI may be derived by determining the quantization level of the reception SINR. In general, the reception SINR is represented as a ratio of desired signal power to undesired signal power, and the SINR measurements may vary depending on selection of the undesired signal power. A document 3GPP R1-072983, “Channel Quality Indicator (CQI) Reporting for LTE MU-MIMO”, Nokia, TSG RAN WG1 meeting #49bis Orlando, Fla. (US), Jun. 25-29, 2007 discloses calculation examples regarding how the measurements vary.
In the selection of the undesired signal power, the reception quality measurement may be significantly affected depending on whether or how user apparatuses other than a user apparatus of interest are taken into account. For example, it is assumed that two user apparatuses UE2, UE3 other than a user apparatus UE1 of interest and streams S1, S2 and S3 are transmitted from a base station to the user apparatuses UE1, UE2 and UE3, respectively. For convenience sake, it is assumed that the number of interference users considered in reception signal quality measurement at user apparatuses is equal to (m−1) and the number of beams transmitted in a spatial multiplexing manner from several antennas of the base station is equal to n. Taking into account the other user apparatuses UE2, UE3, SINR for the user apparatus UE1 can be formulated as follows,SINR=P1/(P2+P3+N)  (A1).In this case, n=3 and (m−1)=2 where n=m. In the formula, P1 represents reception power of the stream S1 at the UE1, P2 represents interference power with the UE1 due to the stream S2, P3 represents interference power with the UE1 due to the stream S3, and N represents a noise signal component.
If only one of the other user apparatuses is taken into account, the SINR for the user apparatus UE1 can be formulated as follows,SINR=P1/(P2+N) or P1/(P3+N)  (A2).In this case, n=3 and (m−1)=1 where n≠m. In addition, if neither of the other user apparatuses is taken into account, the SINR for the user apparatus UE1 can be formulated as follows,SINR=P1/N  (A3).In this case, n=3 and (m−1)=0 where n≠m.
Pilot signals (or reference symbols) arriving at the user apparatuses are orthogonal to each other. Thus, the reception signal quality of the pilot signals may be evaluated in the formula (A2) or (A3). However, data signals transmitted in PDSCHs are not orthogonal among different users, and accordingly evaluation by means of the formulae (A2) and (A3) may be inaccurate. For data signals, an interference signal component due to signals destined for the other users as represented by the formula (A1) cannot be ignored.
In order to report as accurate measurements of channel states to the base station as possible, all of the other user apparatuses may have to be taken into account, as represented by the formula (A1). In this case, however, different measurement methods for the reception signal quality have to be applied depending on which multiple user MIMO (MU-MIMO) based communication or single user MIMO (SU-MIMO) based communication is utilized. This is the reason why other users are not taken into account for the reception quality measurement in the SU-MIMO scheme. However, such switches of the measurement method for reception signal quality in a user apparatus depending on either of the MU-MIMO scheme and the SU-MIMO scheme being applied increases system complexity and may not be practical. On the other hand, if a common measurement method for reception signal quality is applied to the MU-MIMO scheme and the SU-MIMO scheme, accurate CQIs cannot be comprehended at the base station.